Christian Sames

Photon-by-photon feedback control of a single-atom trajectory

Feedback is one of the most powerful techniques for the control of classical systems. An extension into the quantum domain is desirable as it could allow the production of non-trivial quantum states and protection against decoherence. The difficulties associated with quantum, as opposed to classical, feedback arise from the quantum measurement process—in particular the quantum projection noise and the limited measurement rate—as well as from quantum fluctuations perturbing the evolution in a driven open system. Here we demonstrate real-time feedback control of the motion of a single atom trapped in an optical cavity. Individual probe photons carrying information about the atomic position activate a dipole laser that steers the atom on timescales 70 times shorter than the atom’s oscillation period in the trap. Depending on the specific implementation, the trapping time is increased by a factor of more than four owing to feedback cooling, which can remove almost all the kinetic energy of the atom in a quarter of an oscillation period. Our results show that the detected photon flux reflects the atomic motion, and thus mark a step towards the exploration of the quantum trajectory of a single atom at the standard quantum limit.

Observation of squeezed light from one atom excited with two photons

Single quantum emitters such as atoms are well known as non-classical light sources with reduced noise in the intensity, capable of producing photons one by one at given times. However, the light field emitted by a single atom can exhibit much richer dynamics. A prominent example is the predicted ability of a single atom to produce quadrature-squeezed light, which has fluctuations of amplitude or phase that are below the shot-noise level. However, such squeezing is much more difficult to observe than the emission of single photons. Squeezed beams have been generated using macroscopic and mesoscopic media down to a few tens of atoms, but despite experimental efforts, single-atom squeezing has so far escaped observation. Here we generate squeezed light with a single atom in a high-finesse optical resonator. The strong coupling of the atom to the cavity field induces a genuine quantum mechanical nonlinearity, which is several orders of magnitude larger than in typical macroscopic media. This produces observable quadrature squeezing, with an excitation beam containing on average only two photons per system lifetime. In sharp contrast to the emission of single photons, the squeezed light stems from the quantum coherence of photon pairs emitted from the system. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters.

Feedback Cooling of a Single Neutral Atom

Maxwell realized in a famous thought experiment, which is nowadays known as Maxwells demon, that the thermodynamic parameters of a system can be altered by performing feedback on the systems individual constituents. Here we report on the implementation of a related scheme to cool the motion of a single neutral atom.

Antiresonance phase shift in strongly coupled cavity QED.

We investigate phase shifts in the strong coupling regime of single-atom cavity quantum electrodynamics. On the light transmitted through the system, we observe a phase shift associated with an antiresonance and show that both its frequency and width depend solely on the atom, despite the strong coupling to the cavity. This shift is optically controllable and reaches 140°--the largest ever reported for a single emitter. Our result offers a new technique for the characterization of complex integrated quantum circuits.

Three-Photon Correlations in a Strongly Driven Atom-Cavity System

The quantum dynamics of a strongly driven, strongly coupled single-atom-cavity system is studied by evaluating time-dependent second- and third-order correlations of the emitted photons. The coherent energy exchange, first, between the atom and the cavity mode, and second, between the atom-cavity system and the driving laser, is observed. Three-photon detections show an asymmetry in time, a consequence of the breakdown of detailed balance. The results are in good agreement with theory and are a first step towards the control of a quantum trajectory at larger driving strength.

Quantum optics with a new cavity QED setup

We report on a new cavity QED setup with asymmetric mirrors, optimized for high photon flux. Novel photon statistics and real-time feedback experiments are now possible.

Feedback cooling of a single neutral atom

Maxwell realized in a famous thought experiment, which is nowadays known as Maxwells demon, that the thermodynamic parameters of a system can be altered by performing feedback on the systems individual constituents. Here we report on the implementation of a related scheme to cool the motion of a single neutral atom.

Observation of time-dependent third-order correlations in cavity QED

A lot of information about a physical system can be gained by characterizing the light field that is emitted. Measurements of second-order intensity correlation functions have become a standard tool to observe the conditional dynamics of various fundamental systems in quantum optics. Here, we present - for the first time - measurements of the time-dependent, third-order intensity correlation function of a genuine quantum system. It consists of a neutral 85Rb atom strongly coupled to a single mode of a high-finesse optical cavity. In order to excite higher-order dressed states of the Jaynes-Cummings ladder we use relatively strong probe powers corresponding to steady state empty-cavity photon numbers as high as 20.